Enrolment

Hello,
I've a problem which makes me crazy for a lot of days.
I use external enrolment with an external table named 'enrol' which contains about 40 000 entries.
Its fields are idnumber_etu, idnumber_course and role.
This worked great with Moodle 1.6.x
The problem, in 1.7.x, is that enrolments take a lot of time and memory.
So, about 3 hours after the beginning, syncing stops with an "Out of memory" error on my Fedora (not in php), why ? recordsets seems to be closed properly.
I'm currently trying to "over-size" the memory of my test server ... but it swaps and crashes.

Thanks for help.